Thorium-234 derived information on particle residence times and sediment deposition in shallow waters of the south-western Baltic sea

Activities of the naturally occurring, short-lived and highly particle-reactive radionuclide tracer Th-234 in the dissolved and particulate phase were measured at three shallow-water stations (maximum water depths: 15.6, 22.7 and 30.1 m) in Mecklenburg Bay (south-western Baltic Sea) to constrain the time scales of the dynamics and the depositional fate of particulate matter. Activities of particle-associated (> 0.4 mu m) and total (particulate+dissolved) Th-234 were in the range of 0.08-0.11 dpm L-1 and 0.11-0.20 dpm L-1, respectively. The activity ratio of total Th-234 and its long-lived and conservative parent nuclide U-238 was well below unity (range: 0.09-0.19) indicating substantial radioactive disequilibria throughout the water column, very dynamic trace-metal scavenging and particle export from the water column at all three stations. For the discussion the Th-234 data of this study were combined with previously published water-column Th-234 and particulate-matter data from Mecklenburg Bay ( Kersten et al., 1998. Applied Geochemistry 13, 339-347). The resulting average vertical distribution of total Th-234/U-238 disequilibria was used to estimate the depositional Th-234 flux to the sediment. There was a virtually constant net downward flux of Th-234 of about 28 dpm m(-2) d(-1) leaving each water layer of one meter thickness. Thorium-234-derived net residence times of particulate material regarding settling from a given layer in the water column were typically on the order of days, but with maximum values of up to a couple of weeks. Based on an average ratio of particulate matter (PM) to particle-associated Th-234 a net flux of about 145 mg PM m(-2) d(-1) was estimated to leave each water layer of one meter thickness. The estimated cumulative water-column-derived particulate-matter fluxes at the seafloor are higher by a factor of about 2 than previously published sediment-derived estimates for Mecklenburg Bay. This suggests that about half of the settling particulate material is exported from the study area and/or subject to processes such as mechanical breakdown, remineralisation and dissolution. Lateral particulate-matter redistribution and particle breakdown in the water column (as opposed to the sediment) seem to be favoured by (repeated) particle resuspension from and resettling to the seafloor before ultimate sedimentary burial. The importance of net lateral redistribution of particulate material seems to increase towards the seafloor and be particularly high within the bottommost few meters of the water column. (C) 2008 Elsevier B.V. All rights reserved